1. Field of the Disclosure
The present disclosure is related to the field of downhole nuclear magnetic resonance (NMR) investigation of earth formations. In particular, the disclosure relates to an apparatus and method for methods for simultaneously making acoustic and Nuclear Magnetic Resonance measurements in a borehole of limited diameter.
2. Description of the Related Art
One of the most useful measurements of an earth formation made in measurement-while-drilling operations is that of porosity of the earth formation. The porosity is a measure of the hydrocarbon bearing capacity of the formation as more porous rocks have the capability of having more hydrocarbons.
Porosity measurements are commonly made using a neutron-porosity tool. Making measurements with a neutron-porosity tool requires the use of a nuclear source. In some applications, such as in drilling boreholes of small diameter (slimhole), space and safety considerations may preclude the use of a nuclear source. The term “slimhole” is defined in the Schlumberger Oilfield Glossary as:                An inexact term describing a borehole (and associated casing program) significantly smaller than a standard approach, commonly a wellbore less than 6 in. in diameter. The slimhole concept has its roots in the observed correlation between well costs and volume of rock extracted. If one can extract less rock, then well costs should fall. One form of slimhole work involves using more or less conventional equipment and procedures, but simply reducing the hole and casing sizes for each hole interval.        
The present disclosure describes a tool capable of providing two independent estimates of formation porosity in a slimhole.